Many different forms have been proposed for a tool holder which will accept a tool or tool adapter in a socket and lock it in place. U.S. Pat. No. 2,107,581 suggests a construction wherein a spring-urged wedge moves a detent laterally toward engagement with a tool and a socket, with the tool capable of being pulled out of the socket when the wedge is released. U.S. Pat. No. 2,136,190 proposes a screw which acts on a detent member to force it into engagement with a tool in the socket, and the detent is spring-urged toward disengagement. U.S. Pat. No. 2,500,420 shows a screw which acts to move a pawl laterally into engagement with a punch in a socket. A difficulty with the latter two patent constructions is that the machinery is often subject to vibration and jarring, which can loosen the screw and cause the tool to come out of the socket, with perhaps disastrous results. In all of these constructions, a pull on the tool will remove it from the socket when the detent is not in the engaged position, because in none of these is there any positive locking action.